Voltage-dependent Na channel a subunit that forms pore is known to include 9 kinds at present. Recently, it has been evidenced that the subunit, particularly Nav 1.7 is broadly concerned in the signal transduction of acute and chronic pain.
SCN9A (Nav 1.7) is tetrodotoxin (TTX)-sensitive Na channel localized in the peripheral sensory nerve or sympathetic nerve, which is also referred to as NENA or PN1. Physiologically, Nav 1.7 channel functions to amplify a pain signal (i.e., generate a generator potential) at the sensory nerve ending. In the field of genetic investigation, it has been getting evident that a human whose SCN9A gene mutates to result in loss-of-function shows congenital insensitivity to pain. Reversely, in patients suffering from a severe orphan disease such as erythromelalgia and paroxysmal extreme pain disorder, it is observed that SCN9A gene mutates to result in gain-of-function. Furthermore, it has been reported that approximately 30% of patients suffering from small fiber neuropathy have genetic polymorphism to enhance Nav 1.7 function (Non-Patent Literature 1). And, it is suggested that Nav 1.7 channel function is directly concerned in the hyperexcitability of DRG neuron in patients suffering from pain since the expression level and activity increase in DRG neuron of model animals suffering from chronic pain, and neuropathic pain and inflammatory pain decrease in a knockout experiment (Non-Patent Literature 2).
Patent Literature 1 discloses a benzimidazole derivative represented by the following formula (A), but the compound have 2-((4-cyclopropylpyridin-2-yl)amino)isonicotinonitrile as an essential partial structure, which is different from the compound of the present invention. And, the invention described in Patent Literature 1 is directed to a Syk tyrosine kinase inhibitor, thus Patent Literature 1 does not disclose the present invention at all.
